The present invention relates to an optical multilevel transmitter which includes a semiconductor quadrature optical modulator for converting an electric signal into an optical signal, and an optical transponder. In particular, the invention relates to an optical multilevel transmitter which includes a semiconductor nonlinear characteristic compensation circuit, and an optical transponder.
Increased use of massive amounts of content such as high-resolution images and explosive use of mobile terminals typified by smartphones have considerably increased Internet traffic. Accordingly, optical communication systems are required to have much larger capacities.
In addition to technologies such as modulation speedup and wavelength multiplexing, there have been developed technologies in recent years that make optical signals multilevel to use frequencies more efficiently. Making optical signals multilevel (hereafter referred to as optical multilevel modulation) is a modulation technology that simultaneously transmits multiple bits of information using a single-symbol optical signal by superimposing phase and polarization information on the optical signal.
Many optical multilevel modulation techniques have been disclosed. R. A. Griffin, et al., “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration,” OFC2002, paper PD-FD6, 2002 discloses quadrature phase shift keying (QPSK). P. J. Winzer, “Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 4, Feb. 15, 2010, pp. 547-556 discloses a 16-QAM signal coherent reception technique involving polarization multiplexing.
FIGS. 1A to 1D are diagrams showing complex phase planes used in optical transmission and signal constellations according to known modulation techniques. In FIGS. 1A to 1D, the points of optical multilevel signals are plotted on a complex phase plane (also called a complex plane, phase plane, or IQ plane) (complex representation of an optical electric field at an identified time).
FIG. 1A shows signal points on an IQ plane. In FIG. 1A, each signal point can be represented by complex rectangular coordinates (IQ coordinates) or polar coordinates consisting of amplitude r(n) and phase φ(n).
FIG. 1B shows quadrature phase shift keying (QPSK), by which 2-bit information (00, 01, 11, 10) is transmitted using a single symbol and using four values (0, π/2, π, −π/2) as phase angles φ(n). FIG. 1C shows 16-quadrature amplitude modulation (16-QAM), which is widely used in wireless communications. In 16-QAM, signal points are arranged in a lattice pattern, and 4-bit information can be transmitted using a single symbol.
FIG. 1D shows 64-QAM signals. In 64-QAM, 6-bit information can be transmitted using a single symbol. On the other hand, a great number of signal points are arranged very densely and therefore performance such as reception sensitivity tends to degrade due to displacement of the signal points, or the like.
Not only optical multilevel modulation techniques but also optical modulators for transmitting optical multilevel signals have been considered in recent years. Current optical multilevel transmitters mostly use a lithium niobate (LiNbO3; hereafter simply referred to as LN) quadrature optical electric field modulator (hereafter referred to as IQ optical modulator). However, LN quadrature optical electric field modulators have challenges: reductions in power consumption and downsizing.
E. Yamada, et al., “112-Gb/s In PDP-QPSK modulator integrated with a silica-PLC polarization multiplexing circuit”, OFC 2012, PDP5A.9, (2012) discloses a semiconductor IQ optical modulator which includes a semiconductor material in place of an LN material. This semiconductor IQ optical modulator can be integrated with a light source such as a semiconductor laser and therefore can be downsized. Further, the semiconductor IQ optical modulator can obtain changes in refraction index more efficiently than LN modulators by using the quantum-confined Stark effect of a multiple quantum well structure serving as a refraction index change mechanism which utilizes application of an electric field. Thus, it can reduce power consumption.